Not only the fact that a multiplayer mod is similar to their own proprietary service but what also has to be considered is that it's their game. Creating a mod for it is theoretically speaking never legal, they have the right to ban all modding, singleplayer and multiplayer if they desire to do so. The game is their intellectual property.

Maybe something like this could be used in the final version as sort of the JC3MP equivalent for Minecraft's Dynmap? With some serverside scripting I guess it should be possible to show live locations on this when maybe modifying it a bit.

Hi, I'm Btje!
I'm a 22 year old male from The Netherlands. I used to be a beta tester on GTA MP and I am very happy JC3 was chosen as its replacement.
I can't wait for the great things to come and I can't wait to test this too!

Back when playing SA:MP I mostly played on RP servers but personally I enjoyed making stunt scripts so I will most likely create a stunt script for this as well. I don't know whether this will be 24/7 or even public but we'll see.
I've also been thinking, wouldn't it be fun to have a server that was created by the community? Perhaps via github so everyone can contribute. I've been playing with this idea but I'm not sure it will work because there will most likely be trolls but it would be fun to see what it would turn out to be.